The subpoenas from Representative Richard E. Neal, Democrat of Massachusetts, to Treasury Secretary Steven Mnuchin and Charles P. Rettig, the I.R.S. commissioner, amounted to an unexpected shift in tactics in the yearslong Democratic effort to secure tax returns that Mr. Trump has refused to release. Mr. Mnuchin had rejected a request for the returns made under a little-known provision of the federal tax code that dates back to the Teapot Dome scandal of Warren G. Harding’s administration nearly a century ago.

So Mr. Neal is turning to a more conventional avenue: the subpoena.

“After reviewing the options available to me, and upon the advice of counsel, I issued a subpoena today to the secretary of the Treasury and the commissioner of the I.R.S. for six years of personal and business returns,” Mr. Neal said in a statement. “While I do not take this step lightly, I believe this action gives us the best opportunity to succeed and obtain the requested material.”

The new approach is unlikely to be any more fruitful in the short term, given Mr. Trump’s vow to fight all subpoenas from House Democrats. Subpoenas are now pending from the Ways and Means, Judiciary, Oversight and Reform, Financial Services and the Intelligence Committees.

But like those other broiling disputes between the legislative and executive branches, the fight over Mr. Trump’s tax returns could soon head to the federal courts, and House lawyers believe they have a better and speedier case defending a subpoena than a legally untested tax code provision.

Mr. Neal gave the agencies until 5 p.m. May 17 to produce the records, and is expected to go to court to enforce his request shortly thereafter if the Trump administration continues to argue that the committee does not have a legitimate legislative purpose that warrants compliance.

The subpoena requests six years of Mr. Trump’s personal tax returns as well as returns for eight other entities that represent the core of his business empire from 2013 to 2018. It also demands any related paperwork but, unlike the earlier request, does not seek information on potential audits conducted in those years.

The Treasury Department and the I.R.S. did not immediately comment on the subpoenas. Even if they were to comply, it is unlikely the tax information would become public anytime soon given legal concerns about taxpayer privacy.

Mr. Neal had been weighing whether to file a lawsuit immediately to try to enforce the century-old tax code provision he relied on initially or to shift course and issue a subpoena to compel production of the returns.

His initial invocation of the tax code provision — Section 6103 — was novel and set off a flurry of legal scholarship. The section gives the chairmen of Congress’s tax-writing committees unique powers to request information on any taxpayer from the I.R.S. and says only that the executive branch “shall” furnish the material upon request. The House Ways and Means Committee and Senate Finance Committee routinely use this method to study the effects of tax policy on real taxpayers, but there is scant precedent for using it to study a president’s tax information — a fact that Republicans seized on to oppose the effort.

Ultimately, Democrats decided it posed too many risks to bring directly to court.

Democrats have clamored for years for Mr. Trump to release his returns, as every other modern presidential nominee had done before him. They have speculated that the president is trying to hide details of his actual financial worth as well as information that could reveal conflicts of interest between his businesses and his governmental responsibilities. Some Democrats argue they could also reveal evidence of tax evasion.

The requests infuriated the president and his family. Mr. Trump, his personal lawyers and his Republican allies say that rationale is absurd and that Democrats are clearly concocting a fake investigation to justify getting ahold of information they can use to politically embarrass the president.

Mr. Trump’s lawyers have already formalized those arguments in two lawsuits meant to stop companies that have worked with the president from handing over financial information subpoenaed by other House committees.

But it was up to Mr. Trump’s government, not his private lawyers, to decide on the Ways and Means request. After weeks of delays, Mr. Mnuchin formally rejected the request in writing on Monday, telling Mr. Neal that after consulting Treasury and Justice Department lawyers, he had determined that the request “lacks a legitimate legislative purpose” and that he was not authorized to disclose the information in question.

“As you have recognized, the committee’s request is unprecedented, and it presents serious constitutional questions, the resolution of which may have lasting consequences for all taxpayers,” Mr. Mnuchin wrote. He added that the department would be willing to provide the committee more generalized information about how the I.R.S. conducts presidential audits.

Mr. Neal’s decision to abandon his earlier approach and issue a subpoena for the material suggests that the dispute over the power of 6103 could go left unresolved by a court for now. In a lengthy letter to Mr. Mnuchin and Mr. Rettig accompanying the subpoenas, the chairman maintained that his earlier request had been valid and asserted that he viewed the agencies as out of compliance with the law.

An aide to the Ways and Means Committee suggested on Friday that though the panel was confident in the merits of the chairman’s argument, Mr. Neal and lawyers for the House ultimately decided that going straight to court to try to enforce that provision carried too much risk. They feared that a judge could simply throw the case out for lack of standing, essentially rendering use of the tax code provision unenforceable whenever the executive branch objects to its invocation.

Regardless, legal experts say a court case would most likely turn on questions about the expansiveness of Congress’s investigative powers. Mr. Trump’s team has repeatedly made the case that the House’s oversight must be strictly tied to possible legislation. Democrats say court cases make clear their authority is broad.

A version of this article appears in print on , on Page A15 of the New York edition with the headline: Democrats Try New Approach to Get Trump’s Tax Returns: The Subpoena. Order Reprints | Today’s Paper | Subscribe